In the textile industry, it is often necessary to treat yarns, particularly to increase their wettability with respect to dyeing baths or other chemical substances. The word "yarn" as used in the present specification must be understood with a wide sense, including natural fibers, for example cotton, and synthetic fibers, in monofilament and multifilament form.
It is old in the art to subject a yarn to an electrical discharge. More precisely, the yarn is passed through an AC discharge between a dielectric-coated cylinder and a metal plate. Such a treatment has drawbacks. It requires considerable electric power. The fiber must remain several seconds in the discharge if a modification of the fiber is to be obtained. Such a time duration is incompatible with in-line yarn treatments.
In another prior art process (British Pat. No. 1,300,088), the yarn is subjected to an electric arc which is rotated so that the yarn is subjected repetitively to the arc but each time for a short moment. It will be explained later that that approach too has shortcomings.
It is an object of the invention to provide an improved process which requires less consumption of energy and allows high-speed travel of the yarn.
For that purpose, the yarn is passed through a location where an arc is struck and cut at high frequency. Interruption of the arc may be by way of an impedance placed in the electric circuit creating the arc. The time period of repetition will be higher than the duration of an individual arc by at least one and typically several orders of magnitude.
The effect of the interrupted arc, which may be compared to a spark, is very different from that of a sustained arc; this difference may conceivably be attributed to the fact that the interrupted arc strikes at a much higher voltage (at least one order of magnitude) than the permanent voltage of a sustained arc and impresses a much greater energy to the charged particles. The excitation spectrum (spark spectrum) is much richer and the energy levels much higher than in a sustained arc. Furthermore, the use of an interrupted arc avoids the problem created by the use of a sustained arc; due to quenching of the arc, there is no "pick-up" at a point of the moving yarn.
It will in general be necessary for the average current to be at least 400 .mu.A for a yarn travelling at 5 m/min and to be all the higher the higher the speed of the yarn. The average value of the arc current may further be controlled responsive to the travelling speed of the yarn.
The risk of burning and cutting the yarn is avoided because each discharge is extremely brief; the high repetition frequency of the arcs allows moreover the yarn to be treated over the whole of its length. Each discharge has a high peak power, but involves a low energy.
The process will generally be carried out in air when the only object is improved wettability, since it has the advantage of simplicity. The arc may be fed with DC current or rectified AC current; the second embodiment has the advantage of extinction at each return to zero voltage.
A treatment device according to another aspect of the invention comprises at least one module formed from a first electrode and a second electrode confronting each other at a predetermined distance. An electrical circuit is provided for applying an arc striking voltage between the electrodes. Driving means cause the yarn to travel between the electrodes through a location where the arc appears. The circuit comprises a generator capable of establishing between the two electrodes an arc voltage in the absence of current flow in the circuit, the power of the generator and the impedance of the circuit being such that the arc is interrupted, after striking, in a short time with respect to the rise time to the striking voltage of the arc.
Striking of the arc elsewhere than at the location of the yarn must be avoided. For that, the yarn path may be straddled by dielectric material inducing the arc to strike between the electrodes at the location of the yarn. However, this precaution has generally proved to be superfluous.
In another embodiment, the electrodes are in the form of plates or blades parallel to a same direction and at an angle so as to be closest at the location where the yarn passes.
When it is desired to work with a high yarn speed, it is advantageous to subject this latter to several successive arcs. For that, the device may comprise several modules disposed successively along the path of the yarn and corresponding to several different directions of the arc paths about the yarn. Instead of providing several modules, the yarn may be caused to travel in the form of a coiled winding, whose adjacent turns are in contact or very close to one another and travel under the same plate-shaped electrode.
The invention will be better understood from the following description of particular embodiments given by way of examples only.